//
// Created by wze-zero on 2024/7/9 009.
//
#include <iostream>

using namespace std;

int main() {
    int n, sum;
    cin >> n;
    for (int i = 0; i <= n; ++i) {
        if (i % 2 == 0) {
            sum += i;
        }
    }
    cout << sum << endl;
    return 0;
}
